1. Field of the Invention
This invention relates to a multiplex transmission system for carrying out a data transmission among a plurality of multiplex nodes connected to a common multiplex transmission line, and more particularly, to a multiplex transmission system using CSMA/CD (carrier sense multiple access/collision detection).
2. Description of the Related Art
In conventional multiplex transmission systems of this type, a plurality of terminals (multiplex nodes) are connected to a common multiplex transmission line (multiplex bus) formed of, e.g., a paired cable. One of the multiplex nodes sends a predetermined data frame to the multiplex bus in accordance with the CSMA/CD method, to thereby transmit data simultaneously to the other multiplex nodes. The data frame includes an acknowledge signal (ACK signal) area at an end thereof.
In such systems, it is known that each of the multiplex nodes that received the data frame returns an ACK signal to a bit position of the ACK signal area preassigned thereto.
Each multiplex node has a communication control device (LSI), a control circuit (CPU), which is a computer for communication, and an input/output interface (I/F) circuit.
The CPU fetches data indicating events from various switches and the like through the I/F circuit connected thereto, and the LSI (large-scale integrated circuit) of each multiplex node operates independently of those in the other multiplex nodes. The LSI includes a transmission buffer memory for storing message data supplied from the CPU and a data ID identifying this data, and is able to send the data frame of this message to the multiplex bus at desired timing. Therefore, a collision of transmitted messages can occur on the multiplex bus, and conventionally this is prevented by setting a priority sequence for the transmission of individual messages.
In accordance with the priority sequence thus set, each LSI carries out a priority control to send a message without destroying same. Specifically, the LSI carries out a data transmission according to a nondestructive arbitration type CSMA/CD access method in which the transmission of a message with a lower priority is automatically interrupted and only the higher-priority message is continuously transmitted. When the data is properly received, each multiplex node returns an ACK signal to a location of address uniquely assigned thereto (the respective bit area in the ACK signal area).
If a local error occurs in the network and the data received by a multiplex node is erroneous, then no ACK signal is returned from this multiplex node, and therefore, the sending multiplex node determines that an abnormal situation occurred, and retransmits the data. The sending multiplex node repeats the transmission of the data, e.g., three times at the maximum, until all of the registered multiplex nodes return the ACK signals. In prior art systems, all nodes have the function of determining whether or not the ACK signals have been returned from the nodes and retransmitting data. To achieve this function, each node has a 24-bit memory, for example, and all of the nodes connected to the multiplex bus are registered in this memory.
A sending multiplex node uses an ACK management function, i.e., if a node fails to return the ACK signal even though the data transmission was repeated three times, such a node is regarded as being in trouble and is excluded from the registration in the memory, and when an extra ACK signal is received, the node corresponding to this ACK signal is registered.
In a multiplex node in which data supplied thereto has changed due to a change of events, the CPU writes transmission data in the transmission buffer memory of the LSI and generates a send request, and upon receiving the send request, the LSI starts to send the transmission data. After the transmission by the LSI is completed and the transmission buffer memory becomes empty, the CPU can write next transmission data. In some multiplex nodes, if the need to transmit new data arises before the transmission is not yet completed, the request to send the data already written in the transmission buffer memory is canceled, to allow the new data to be written therein.
In the multiplex transmission system described above, all of the nodes in the network have the ACK management function, and therefore, a large-scale communication control circuit must be used. Further, each multiplex node must determine the reception states of the nodes including not only the destination node but the other nodes, and accordingly, the occupation time of the multiplex bus is prolonged and the manufacturing cost of the system is increased.
Furthermore, in this multiplex transmission system, when the data loaded into the transmission buffer memory is in a stand-by state, up-to-date data cannot be transmitted unless the transmission of the data in the transmission buffer memory is ended, even through an event change has occurred. Accordingly, the transmission of up-to-date data having greater importance is delayed.
In the above multiplex transmission system, moreover, when the request to send the data stored in the transmission buffer memory is canceled, the stored data is lost. Therefore, to retain such data, an additional buffer memory must be provided for the queue, which entails an increase of the manufacturing cost.